1. Field of the Invention
The present invention relates to a tool changing mechanism to be incorporated into, for example, a printed-circuit board drilling machine, and a tool pressing mechanism included in the tool changing mechanism.
2. Description of the Related Art
A known tool changing mechanism disclosed in JP No. 4-2437A and employed in a printed-circuit board drilling machine has a tool magazine comprising a plurality of tool posts arranged in rows, and a plurality of pressing cylinder actuators arranged so as to correspond to the tool posts, respectively. The tool magazine including the pressing cylinder actuators is fixedly held on a movable work table. The pressing cylinder actuators are controlled individually by a controller. The position of the movable work table with respect to a cross feed direction and the position of the spindle of the printed-circuit board drilling machine with respect to a longitudinal feed direction are adjusted to locate a desired one of the tool posts held in the tool magazine in alignment with the spindle. Then, the controller controls the pressing cylinder actuator corresponding to the desired tool post to transfer a tool from the tool post to the spindle or to transfer a tool from the spindle to the tool post.
Another known tool changing mechanism disclosed in JP 4-122537A and employed in a printed-circuit board drilling machine has a tool magazine comprising a plurality of tool posts, and a plurality of pressing cylinder actuators arranged so as to correspond to the tool posts, respectively. All the pressing cylinder actuators are controlled for simultaneous operation by a controller. The pressure to be exerted on the tool by each pressing cylinder actuator is adjusted so that the pressing rod of the cylinder actuator is moved back when the spindle of the printed-circuit board drilling machine exerts a pressure on the shank of a tool held on the tool post corresponding to the pressing cylinder actuator.
Each of those known tool changing mechanisms is provided with a plurality of pressing cylinder actuators respectively corresponding to a plurality of tool posts held in the tool magazine. Therefore, those known tool changing mechanisms need a large number of parts including O-rings, pistons and sealing members, need much time for the assembling work and are costly. The pitches of the tool posts are dependent on the diameter of the cylinders of the pressing cylinder actuators. Since each pressing cylinder actuator exerts a pressure directly on the spindle when transferring the tool between the tool post and the spindle, a large axial load is exerted on bearings, such as pneumatic bearings employed in the printed-circuit board drilling machine, supporting the spindle for rotation, which is possible to reduce the accuracy of rotation of the spindle. Accordingly, a special cylinder actuator having a cylinder of a small diameter and a small pressing capacity has been used as the pressing cylinder actuator to avoid exerting an excessive axial load on the bearings supporting the spindle. Such a special cylinder actuator, however, is expensive and increases the cost of the tool changing mechanism. Since the movable work table holding the tool magazine including the plurality of pressing cylinder actuators is heavy and hence the same cannot be rapidly moved.
The controller of the former known tool changing mechanism needs to control the pressing cylinder actuators individually. Therefore, the controller has a complicated configuration and needs complicated piping. The controller of the latter known tool changing machine does not control the pressing cylinder actuators individually, which is the advantage of the latter known tool changing machine over the former tool changing machine. However, since the all the pressing cylinder actuators are actuated simultaneously in the latter known tool changing machine, the latter known tool changing machine consumes more compressed air than the former known tool changing machine, which is economically disadvantageous.
Accordingly, it is an object of the present invention to provide a tool changing mechanism provided with tool pressing mechanisms not subject to many dimensional restrictions, not fixedly held on a movable work table to enable the movable work table to be moved rapidly, and capable of being easily controlled for tool changing operations and of being manufactured at a low manufacturing cost.